This invention relates to a method for initiating an in-situ combustion process to recover energy raw materials such as petroleum hydrocarbons from a subterranean formation by the introduction of oxygen into the formation.
Since the invention of the in-situ combustion method for petroleum recovery by F. A. Howard in 1923, a number of methods have been developed, the object of which is the production of heat within the reservoir, especially of sufficient heat, by means of partial combustion of oil residues in a petroleum reservoir to enable recovery of the remaining oil. The most important mechanisms contributing to enhanced recovery are viscosity reduction by means of heat, distillation and cracking of the oil and of the higher boiling components, sweeping out of the oil with hot water and extraction of the oil by means of miscible products.
The use of high purity oxygen in place of air significantly improves the performance of the in-situ combustion process. One of the disadvantages of the use of oxygen is its hazardous nature that could lead to uncontrolled reactions or explosions. Because of the hazardous nature of pure oxygen in reacting with other materials much work has been done to reduce this danger. In addition to the question of reaction of oxygen with various materials, the dynamics of compressible fluids is also an important factor in determining what hazard exists when a material is reacted with oxygen.
It is known from experience in autogenous gas cutting that not only the nature of the material but also the composition of the gas used has an influence of the material's cutting quality. With an oxygen content of less than 95%, steel can still be ignited but combustion is not self-sustaining. These ratios apply to atmospheric pressure. In one series of tests, Hvizdos et al, (Journal of Petroleum Technology, June 1983, pp. 1061-1070), reported that samples of carbon steel with a geometry similar to the tubing used in injection wells were tested. The results of Hvizdos et al show that oxygen concentration and pressure have a dramatic effect on flame propagation. For instance, it was found that the tubing, once ignited, would not continue to burn if the oxygen concentration was below a critical level but that the flame would propagate if the oxygen concentration was over that level. The critical level of oxygen concentration is a function of pressure, illustrating that data at low pressure should not be used to plan projects which will operate at high pressure. For safety, a low limit of 45% oxygen should be used for a wide-range of pressures.
Great importance is accordingly attached to the structure of the spaces in which the oxygen is flowing. Should said spaces possess a large inner surface in relation to the volume, then the danger of an explosion when a fuel and oxygen are reacted is greatly reduced. Consequently the reaction of oxygen with oil contained in the pores of the reservoir rocks poses relatively few problems. However, given certain geometric proportions of the spaces through which the oxygen flows, local temperature peaks can occur and cause ignition of the material (steel, plastic, wood, etc.). It follows that the most dangerous point along the oxygen's flow path is the borehole. The operating conditions in a petroleum borehole are such that when high percentage oxygen is introduced there is a great danger of an explosion in the borehole. Neither is the borehole equipment made from deflagration-proof material (copper, Inconel) nor is the condition of the equipment, due to contact with corrosive, erosive and organic agents, such that the danger is lessened.
The injection of oxygen into a wellbore presents significant hazards and requires safety precautions. Previous work in this regard includes the injection of O.sub.2 through a bottom water zone, as disclosed in U.S. Pat. No. 3,208,519 by T. V. Moore, and the initiation of combustion with air followed by oxygen as disclosed in U.S. Pat. No. 4,042,026 by G. Pusch et al. All these methods use air to establish gas flow. However, it has been found that injection of air increases the viscosity of the oil by 100 times when the oil is contacted by air for two days. This increase in viscosity is detrimental to the recovery process.
U.S. Pat. No. 1,410,042 to Shu discloses an in-situ combustion operation wherein a mixture of oxygen and carbon dioxide is injected into the formation to initiate combustion followed by injection of oxygen.
It is therefore the objective of this invention to eliminate these risks or at least reduce them to an acceptable level within the framework of conventional equipment used in boreholes for the recovery of energy raw materials such as petroleum hydrocarbons.